Air-conditioning unit and method

ABSTRACT

An air-conditioning unit including an air circuit with an air inlet, a main fan and an air outlet designed to be connected to a chamber, preferably via one or more flexible ducts, and a refrigerant circuit including a heat exchanger/evaporator positioned in the air circuit to cool the air by evaporating the refrigerant, a compressor and a condenser for condensing the refrigerant before it is returned to the heat exchanger/evaporator. The heat exchanger/evaporator includes several parallel circuits each having at least one regulator valve. The air circuit also includes a temperature probe downstream of the heat exchanger/evaporator and connected to a controller which controls the regulator valves to regulate the flow of refrigerant, and a pressure probe at the air outlet and connected to a regulator for regulating the speed and/or the power of the main fan so as not to exceed a maximum raised pressure at air outlet.

The present invention relates to an air-conditioning unit and method forenclosures, such as aircraft parked on the ground.

It is often necessary to make conditioned air reach enclosures in orderto maintain suitable parameters for the comfort of persons and animals,the functioning of equipment and/or the preservation of goods.Conditioned air means air where at least the temperature is maintainedin a range substantially independent of the temperature of the ambientair. In particular, in aircraft parked on the ground, if onboardair-conditioning systems are not in operation, the temperature on boardmay rapidly become very uncomfortable for the passengers as well as thecrew and/or damage onboard equipment, such as the onboard electronics.To prevent this, it is well known to persons skilled in the art to useair-conditioning units, possible mobile and/or autonomous, in order tobring conditioned air inside the aircraft, in particular throughflexible ducts connecting the aircraft to at least one air outlet of theconditioned air unit, in order to maintain a suitable temperature in thecabin, and/or in the baggage and/or avionics holds. Suchair-conditioning units have been described for example in the Americanpatents U.S. Pat. No. 5,031,690 and U.S. Pat. No. 5,099,652, in theGerman patent application DE 33 14 763 A1, in Japanese patentapplication JP 10-122607 and in the American patent application US2004/0045308 A1.

In particular, U.S. Pat. No. 5,031,690, U.S. Pat. No. 5,099,652 and DE33 14 763 A1 disclose air-conditioning units comprising:

-   -   an air circuit with an air inlet, a main fan and an air outlet        adapted to be connected to an enclosure, such as for example an        aircraft on the ground, preferably through one or more flexible        ducts, and    -   a refrigerant circuit comprising an exchanger/evaporator placed        in the said air circuit in order to cool the air by evaporating        the refrigerant, a compressor and a condenser for condensing the        refrigerant before it is returned to the exchanger/evaporator.

In these units, an air-conditioning method may be implemented comprisingthe following steps:

-   -   ambient air enters an air circuit propelled by a main fan,    -   the said ambient air passes through an exchanger/evaporator,        where it is cooled by evaporating a refrigerant flow circulating        in the exchanger/evaporator,    -   the said cooled air is propelled through an air outlet to a        substantially closed space, such as for example an aircraft on        the ground, preferably through one or more flexible ducts,    -   the said refrigerant it compressed in a compressor downstream of        the exchanger/evaporator,    -   the said refrigerant is condensed in a condenser downstream of        the compressor.

However, all these air-conditioning units have the drawback of beingable to adapt the refrigeration power only within very limited ranges.Often, as described in the introduction to U.S. Pat. No. 5,099,652, thetemperature of the air is simply regulated by alternating between afull-power refrigeration position and a zero refrigeration position.Although in this document, as well as DE 33 14 763 A1, it is proposed toregulate the refrigeration power by varying the power of the compressor.However, the efficacy of this regulation is limited by the fixedparameters of the exchanger/evaporator. For this reason, the existingair conditioning units and methods have limited adaptability tovariations in environmental parameters, such as the temperature andhumidity of the ambient air. This adaptability is also compromised bylow efficiency outside optimum operating parameters and by the necessityto make complex manual adjustments in order to regulate therefrigeration power.

Another very important drawback of the conditioned air units of theprior art is their lack of adaptability to the parameters of theenclosures for which the conditioned air is intended. Each enclosure ischaracterised by an output/pressure curve that is particular to it. Thischaracteristic may be fixed or change over time. The output/pressurecurve of an enclosure may vary, for example, according to the size ofthe enclosure, other characteristics particular to the enclosure,circumstances of the connection environment, and certain variables overtime, such as the opening or modification of the air distributionducting in an enclosure such as an aircraft parked on the ground.However, the air-conditioning units of the prior art do not have meansfor adapting to such variations in the output/pressure curve.

This lack of adaptability is particularly problematic forair-conditioning units for aircraft parked on the ground. These aircraftmay have extremely varied volumes and therefore very divergent flowrate/pressure curves. Each aircraft therefore has a set ofcharacteristics that are particular to it in terms of flow rate andpressure. An air-conditioning unit intended for a small aircraft, suchas for example a business jet, will not be able to deliver a conditionedair flow rate sufficient for a large aircraft, such as a large-capacityaircraft. Conversely, an air conditioning unit intended for alarge-capacity aircraft will have a ventilation power such that, if itwere connected to a business jet, the blow back of the small fuselagewould cause an overpressure both in the business jet and in the flexibleconnection ducts such that they might be damaged.

In addition, in one and the same aircraft, this flowrate/pressurecharacteristic curve may change during the functioning of the unitaccording, for example, to a movement of the flexible connection ductsbetween the air conditioning unit and the aircraft, or by the openingand/or closing of valves on the internal air distribution system of theaircraft.

As the refrigeration power must be adapted to the flow rate ofconditioned air, the reduced variability of the refrigeration power inthe conditioned air units also determines their adaptability toenclosures with different flowrate/pressure curves.

Consequently, with the conditioned air units of the prior art, eachairport must have a whole range of air-conditioning units specificallyadapted to each size and even to each model of aircraft. Given the costof each unit, this may therefore represent a very large amount ofimmobilised capital, with a considerable impact on the profitability ofan airport.

An object of the present invention is therefore to provide an airconditioning unit and method with great adaptability both to theenvironmental parameters and to the different flowrate/pressure curvesof the enclosures to be cooled without compromising the efficacy of thecooling.

To achieve this, in the air conditioning unit of the invention:

-   -   the said exchanger/evaporator comprises several parallel        circuits each with an individual pressure reducing valve,    -   the said air circuit also comprises a temperature sensor        downstream of the exchange/evaporator connected to means of        controlling the said individual pressure reducing valves in        order to regulate the flow rate of refrigerant in order to        maintain the air temperature downstream of the        exchanger/evaporator within a set range, and a pressure sensor        at the air outlet connected to means of regulating the speed        and/or power of the main fan in order not to exceed a maximum        overpressure at the air outlet, and    -   the said refrigerant circuit also comprises a compressor        aspiration pressure sensor connected to means of regulating the        mass flow rate of the compressor in order to maintain the        aspiration pressure of the compressor within a set range.

In this way, the refrigeration power can be regulated through the flowrate of refrigerant in the exchanger/evaporator as a function of the airtemperature downstream of the said exchanger/evaporator. As the saidcontrol means can open only the pressure reducing valves solely in thecircuits of the exchanger/evaporator necessary for achieving thenecessary refrigeration power, the flow rate of refrigerant in each ofthe open circuits can remain close to the optimum point. Under suchconditions, the regulation of the mass flow rate of the compressor inorder to maintain the aspiration pressure of the compressor within anoptimum operating range makes it possible to reduce the refrigerationpower delivered by the compressor when the pressure reducing valves ofthe exchanger/evaporator are closed. With the compressor working at lowoutput, the energy consumption can be reduced. This regulation of themass flow rate of refrigeration aspirated by the compressor can beoperated in several ways. For example, in the case of a rotarycompressor, such as for example a screw compressor, the flow rate can beregulated through the speed of rotation of the compressor. In screwcompressors, it is also possible to regulate the flow rate by action ona capacity slide of the compressor modifying the useful length of thescrew. Another way of regulating the flow rate, if the compressorcomprises several compressor units installed in parallel, is by theregulation of the number of compressor units in operation. In a similarmanner, in the case of a piston compressor comprising several cylinders,where each cylinder comprises at least one induction valve and at leastone exhaust valve, it is possible to regulate the output of thecompressor by acting on the said induction and exhaust valves, in amanner known to persons skilled in the art. Finally, it is also possibleto regulate the output of the compressor by means of a bypass valve,generally external to the compressor.

In addition, the speed and/or power of the main fan can be regulated soas to produce the maximum conditioned air flow rate for each enclosure.As the temperature of the air downstream of the exchanger/evaporatorwill depend both on the refrigeration power and the flow rate of air tobe cooled, this regulation, in combination with the regulation of theindividual pressure reducing valves and the mass flow rate ofrefrigerant sucked in by the compressor cited above has the advantageouseffect of automatically adjusting not only the flow rate of air but alsothe refrigeration power according to each space. It therefore becomespossible, for example, to use the same air-conditioning unit for a wholerange of aircraft of different sizes, without having to proceed withcomplex manual adjustments for each appliance and keeping goodefficiency in all circumstances.

Advantageously, the said air circuit also comprises a temperature sensorat the air outlet connected to means of regulating the speed and/orpower of the main fan in order to control its flow rate so as tomaintain the temperature at the air outlet within a set range. Thismakes it possible to regulate the temperature of the conditioned airthrough its flow rate. Advantageously, the said condenser is anair-cooled condenser. This option is particularly simple and avoidsmeans, such as for example water reservoirs, which will be difficult toadapt to a mobile and/or self-contained unit.

More advantageously, the said air-cooled condenser may comprise acooling fan and a pressure sensor for the refrigerant connected to meansof regulating the speed and/or power of the cooling fan in order tocontrol the cooling of the condenser so as to maintain the pressure ofthe refrigerant in the condenser within a set range. This makes itpossible to regulate the speed and/or power of the cooling fan in orderto adapt it, amongst other things, to the changes in flow rate ofrefrigerant through the condenser and to maintain the condensationpressure.

Advantageously, the said exchanger/evaporator can also comprise atemperature sensor for the refrigerant at the output of each of the saidparallel circuits connected to the said means of controlling the saidpressure reducing valves in order to regulate the flow rate ofrefrigerant so as to ensure the evaporation of substantially all theflow of refrigerant in the said exchanger/evaporator. This makes itpossible to possible to optimise the flow rate of refrigerant in eachcircuit of the exchanger/evaporator by allowing to flow in each circuitonly the refrigerant that can be vaporised by the heat exchanged.

Even more advantageously, the said exchanger/evaporator can alsocomprise a pressure sensor for the refrigerant at the output of the saidparallel circuits connected to the said means of controlling the saidpressure reducing valves in order to correct the temperature sensed bythe temperature sensor for the refrigerant at the discharge from thesame parallel circuit.

Under conditions of high humidity, an excessive refrigeration powermight cause the formation of frost on the surface of theexchanger/evaporator, interfering with and even blocking the flow of airthrough the exchanger/evaporator and therefore causing a breakdown ofthe air conditioning unit. To prevent this, the saidexchanger/evaporator can advantageously also comprise a sensor for thesurface temperature of the exchanger/evaporator connected to the saidmeans of controlling the said pressure reducing valves in order toregulate the flow of refrigerant in order to prevent the formation offrost on the surface of the exchanger/evaporator. With this sensorconnected to the means of controlling the pressure reducing valves, therefrigeration power can be regulated so as to maintain the temperatureabove a frost formation danger threshold.

Between the condenser and the exchanger/evaporator, heat sources canheat the flow of refrigerant. If the cooling in the condenser isregulated optimally, the temperature of the refrigerant should be justbelow the condensation temperature. A heating or a slight drop inpressure could therefore cause the formation of gas bubbles in therefrigerant that might compromise the optimum functioning of the airconditioning unit. For this reason, the refrigerant circuit mayadvantageously also comprise an economiser circuit downstream of thecondenser, the said economiser circuit comprising a main branch, asecondary branch with a pressure reducing valve, and a heat exchangerbetween the said main branch and the said secondary branch downstream ofits pressure reducing valve, so that, in operation, a secondary flow ofrefrigerant is diverted to the said secondary branch so as to beexpanded in the pressure reducing valve and cool through the heatexchanger a primary flow of refrigerant passing through the main branchin order to prevent the appearance of gas bubbles in the refrigerantupstream of the exchanger/evaporator. By profiting from the evaporationheat of the secondary flow of refrigerant, it is possible to supercoolthe primary flow delivered to the exchanger/evaporator in order toprevent the formation of gas bubbles downstream of theexchanger/evaporator.

Advantageously, in order to provide the electrical supply to the airconditioning unit in self-contained mode, the unit may also comprise athermal engine, preferably diesel, coupled to a generator.

Even more advantageously, the said thermal engine may comprise a coolingcircuit for the engine with:

-   -   a radiator, preferably with a fan, for cooling an engine cooling        fluid;    -   a complementary heat exchanger placed in the said air circuit,        preferably downstream of the said exchanger/evaporator, in order        to heat the air by cooling the engine cooling fluid, and    -   a three-way valve for directing the cooling fluid for the engine        alternately to the radiator or to the complementary heat        exchanger.

The characteristics make it possible not only to discharge the heatproduced by the functioning of the thermal engine but also if applicableto use this heat to heat the air-conditioning flow when the unit isfunctioning in heating mode.

Advantageously, the said air circuit may also comprise electric heatingmeans, preferably downstream of the said exchanger/evaporator, with aset of electric elements, preferably wired in several stages, and atemperature sensor downstream of the heating means connected to means ofcontrolling the said set of electric elements to regulate the heatingpower in order to maintain the air temperature downstream of the heatingmeans within a set range. This allows functioning of the airconditioning unit in heating mode with fine regulation of the heatingpower.

Advantageously, the said refrigerant may be a hydrofluorocarbon,preferably R134a. Such a refrigerant affords good energy efficiency,with good safety and low environmental impact.

Advantageously, the said main fan may be a centrifugal fan. Such a fancan deliver a high flow of air with relatively small size.

Advantageously, the said compressor may be a screw compressor.

Advantageously the said unit may be mobile and autonomous in order forexample to supply conditioned air to mobile platforms, such as aircrafton the ground.

Details concerning the invention are described below illustratively butnon-restrictively, making reference to the drawings.

FIG. 1 depicts a side view of an air conditioning unit according to oneembodiment of the invention.

FIG. 2 depicts a plan view of the same air conditioning unit.

FIG. 3 is a pressure and flow rate diagram.

FIG. 4 depicts a diagram of the refrigerant circuit of the same airconditioning unit.

The air conditioning unit 1 for an aircraft illustrated in FIGS. 1 and 2comprises a chassis 2 mounted on wheels 3, although it may alternativelybe fixed, a body 4 provided with an air inlet 5 with filters 5′, and anair outlet 6 with two discharge orifices 7 to which it is possible toconnect flexible ducts to conduct the conditioned air to an aircraft anda system of valves 7 a, 7 b, 7 c for opening and/or closing each of thetwo air orifices 7, and any diversion of the air to the atmosphere.

The unit 1 also comprises, inside the body 4, a diesel engine 8 coupledto an electrical generator 9 in order to provide the supply ofelectricity to the various elements of the unit 1, a variable-speedcentrifugal fan 10 to distribute air to the aircraft, a refrigerantcircuit 11 (depicted in FIG. 4) with an exchanger/evaporator 12 forcooling the air, electric heating means 13 with a plurality of electricelements, and a complementary heat exchanger 14 connected to the coolingcircuit of the diesel engine 8.

In operation, the air intended for the aircraft is aspirated through theair inlet 5 and filters 5′ by the centrifugal fan 10 that is situatedinside the body 4 and functions in free aspiration. All the panelsforming the body 4 thus form negative-pressure aspiration air duct.

After having passed through the fan 10, the air is directed to apressure-resistant air treatment chamber 15 in which theexchanger/evaporator 12, the electric heating means 13 and thecomplementary heat exchanger 14 are found.

The unit 1 also comprises a tank 16 for collecting condensation waterplaced directly below the exchanger/evaporator 12 in order to collectthe moisture from the aspirated air condensing on theexchanger/evaporator 12.

The cooled or heated air is then sent, via the two discharge orifices 7,to the flexible ducts and the aircraft.

A pressure sensor SP1 continuously controls the discharge pressure ofthe fan 10 during operation. This sensor, by means of a frequencyvariator, makes the rotation speed of the fan 10 vary so that the latterdelivers the maximum output possible without exceeding the maximumauthorised pressure, in order to comply with the mechanical strength ofthe flexible ducts, the internal ducting of the aircraft and the airconditioning unit 1. This is illustrated in the diagram in FIG. 3. Inthis diagram, the vertical axis corresponds to the overpressure P at theair outlet 6 with respect to the pressure of the ambient air, with amaximum allowed overpressure Pmax, and the horizontal axis correspondsto the flow rate D. The curves E1, E2 and E3 are flow rate/pressurecurves for enclosures with different characteristics. The curves V1, V2,V3 and V4 are flow rate/pressure curves for the same main fan 10 atdifferent speeds. If the main fan 10 operates at a constant speedindependently of the characteristics of the enclosure, the conditionedair flow will follow one of the curves V1, V2, V3 or V4. With, forexample, the curve V2 the unit 1 will not be able to deliver a flow ofconditioned air below Dmin,V2, below which the overpressure would exceedthe maximum allowed overpressure Pmax, and would risk causing damage. Onthe other hand, the flow rate could increase only with a verysubstantial drop in the discharge overpressure of the enclosure. Thusthe flow of conditioned air that could be delivered to the enclosuresrepresented by the curves E1, E2 and E3 would be determined by theintersections A1, A2, A3 respectively of these curves with V2. With theregulation of the speed of the main fan 10 as a function of the maximumallowed overpressure Pmax, the flow rate is determined by theintersections B1, B2 and B3 of E1, E2 and E3 with Pmax. As can be seenin the diagram, the flow rates DB1, DB2 and DB3 resulting from this aresubstantially higher than the flow rates DA1, DA2 and DA3 of thefixed-speed fan.

Any modification to the air discharge system before the connection orduring functioning results directly in a modification to the rotationspeed of the fan 10 so that the air pressure remains within anacceptable range. Alternatively, the fan 10 could be power regulated,rather than speed-regulated.

A temperature sensor ST1 controlling the air output to the aircraft alsoinfluences the rotation speed of the fan 10, and/or its power, bypreventing discharge into the aircraft of excessively hot air, incooling mode, or too cold in heating mode. The reduction in the speedand/or power consequently makes it possible to reduce the refrigerant orheating load that the unit must deliver in cold mode or hot mode,respectively, in order to comply with its set value. In this way, theunit 1 can be used outside the design conditions without posing aproblem with regard to the comfort afforded by the level of the airdischarge temperature or the discomfort resulting from an excessivelylow discharge temperature.

The cooling mode of the unit 1 is provided by a refrigerant circuit 11based on the compression cycle and on the direct expansion of therefrigerant fluid. This circuit 11 is illustrated in FIG. 4.

The refrigerant fluid used is an HFC (hydrofluorocarbon) preferablyR134a, in order to obtain better performance coefficients and ensureeasy procurement of the refrigerant fluid, whilst complying withenvironmental constraints.

In cooling mode, the heat is taken off from the air treated by theevaporation of the refrigerant in the exchanger/evaporator 12. Thisheat, added to that produced by the compression in a compressor 17, isdischarged to the environment during the condensation of the refrigerantin an air condenser 18.

The exchanger/evaporator 12 comprises a plurality of parallel circuits19. Each parallel circuit 19 comprises, at its inlet, a pressurereducing valve 20 controlled electronically by a regulator. In this way,in operation, the refrigerant is expanded by one or more of thesepressure reducing valves 20 according to the refrigeration loadnecessary. These pressure reducing valves 20 therefore have theparticularity of adapting the flow rate of refrigerant to therefrigeration load strictly necessary.

The regulator for the pressure reducing valves 20 control severalphysical quantities:

-   -   the superheating temperature of the refrigerant detected by a        temperature sensor ST2 at the discharge from each of the        circuits 19 in order to ensure that all the refrigerant present        in the exchanger/evaporator 12 has evaporated before it is        aspirated into the compressor 17;    -   the air temperature detected by a temperature sensor ST3        downstream of the exchanger/evaporator 12 in order to check that        this remains in the set range fixed;    -   the surface temperature of the exchanger/evaporator 12 detected        by a temperature sensor ST4, which gives information on the        possible formation of frost on the exchanger/evaporator 12;    -   the evaporation pressure detected by a pressure sensor SP2 in        the exchanger/evaporator 12, which allows correction of the        superheating temperature in order to have the exact value        thereof.

At the discharge from the exchanger/evaporator 12, the refrigerantvaporised is aspirated at low pressure and compressed to a high pressureby the compressor 17.

In the embodiment illustrated the compressor 17 is of the screw typewith continuous capacity regulation from 25% to 100%. However, inalternative embodiments, the compressor 17 could be a compressor ofanother type, such as for example a piston compressor.

The compressor 17 is driven by an electric motor integrated in thecompressor 17, preferably in an accessible hermetic housing. This motormay be cooled by the aspirated refrigerant fluid.

A pressure sensor SP3 measures the suction pressure upstream of thecompressor 17 and adapts the speed thereof by means of a regulator sothat the rate of the aspirated flow adapts to the rate of the flowinjected by the pressure reducing valves 20 in the exchanger/evaporator12. In other embodiments, the rate of the flow aspirated by thecompressor 17 could be regulated by other means, such as a capacityslide for varying the useful length of the screw in a screw compressor17, the regulation of the number of active compressing units in acompressor 17 having several of these in parallel, a bypass valve,generally external or, in a piston compressor comprising severalcylinders, the regulation of the number of active cylinders, possibly bycontrolling the induction and/or exhaust valves of the cylinders.

The refrigerant fluid leaving the compressor 17 is directed to thecondenser 18.

The condenser 18 is of the air type. It is placed outside the body 4 andcooled by the ambient air propelled through the condenser 18 by acooling fan 21. The cooling air for the condenser 18 circulates in acircuit substantially isolated at least from the conditioned air circuitin order to prevent the latter being contaminated.

To ensure for the condenser 18 a control allowing the latter to adapt tothe refrigerant flow rate, the cooling fan 21 is controlled by afrequency variator connected to a pressure sensor SP4 (high pressure) inthe condenser 18.

The condensation pressure can therefore be kept stable even underchanging external conditions or during changes to the refrigerant flowrate.

A horizontal liquid reservoir 22 collects the refrigerant condensed inthe condenser 18. A dehumidifying cartridge filter 23 placed at theoutput of the reservoir 22 removes any moisture present in therefrigerant.

Before the refrigerant is directed to the pressure reducing valves 20,it is supercooled in an economiser circuit 24.

This economiser circuit 24 comprises a main branch 25, a secondarybranch 26 with a pressure reducing valve 27, and a heat exchanger 28between the said main branch 25 and the said secondary branch 26downstream of its pressure reducing valve 27. Part of the flow ofrefrigerant is diverted to the secondary branch 26 and vaporised by thepressure reducing valve 27 so as to cool the primary flow of refrigerantin the exchanger 28.

This secondary flow of refrigerant is expanded at an intermediatepressure between the high and low pressure of the compressor 17. Thepressure reducing valve 27 is also electronically controlled. Theprimary flow of refrigerant passes through the exchanger 28 on theliquid refrigerant side, while supercooling by virtue of the evaporationof the secondary flow expanded at the intermediate pressure. The latteris then aspirated directly by the compressor 17.

The supercooling of the primary flow affords greater stability offunctioning of the pressure reducing valves 20 of theexchanger/evaporator 12 while at all times providing a 100% liquid phaseof the refrigerant at their inlet.

Variations in refrigeration loads or changes in external conditions maylead to the appearance of gas bubbles at the inlet to the pressurereducing valves 20, interfering with their functioning. By its presence,the economiser circuit 24 prevents this problem.

The heating mode of the unit 1 is provided by electrical heating means30 comprising a set of electric elements.

The electric heating means 13 are placed in the air treatment chamber 15also containing the exchanger/evaporator 12 downstream of it.

The electric heating means 13 are in the form of a set of stainlesssteel tubes without fins. They are wired in for example four differentstages providing the possibility of delivering the heat capacitystrictly necessary.

The temperature sensor ST1 controlling the output of the unit also makesit possible to regulate the electric means 13.

As the unit 1 of the embodiment illustrated is supplied by aself-contained electricity generator, a complementary heat exchanger 14connected to the cooling circuit of the diesel engine 8 can also heatthe treated air. This complementary heat exchanger 14 is of the typecomprising copper tubes and aluminium fins and serves to recover theheat given off by the cooling circuit of the diesel engine 8.

According to the method of use of the unit 1, in cooling or heatingmode, and the water temperature in the cooling circuit of the dieselengine 8, a three-way valve (not illustrated) in this cooling circuitmakes it possible to direct the water circulating in the circuit to aradiator 29 discharging the heat to the environment or to thecomplementary heat exchanger 14 installed in the air treatment chamber15 downstream of the exchanger/evaporator 12.

The diesel engine 8 is cooled by the circulation of water in its coolingcircuit in the cylinder heads. The radiator 29 is preferably ventilatedby means of an axial fan 31.

To comply with certain environmental constraints, it is preferable touse a diesel engine with combustion air cooling. An exchanger forcooling the combustion air, or intercooler, is then installed in thechassis of the radiator 29.

The diesel engine 8 has the following safety devices:

-   -   water high temperature    -   lack of oil pressure.

An electrical panel comprises the power and control circuit for theentire unit 1.

An automatic controller installed in the electrical panel manages thefunctioning of the unit 1 by controlling all the constituent elements,namely:

-   -   main fan 10,    -   compressor 17,    -   condenser 18,    -   pressure reducing valves 20 of the exchanger/evaporator 12,    -   pressure reducing valve 27 of the economiser circuit 24,    -   electric heating means 13,    -   three-way valve of the cooling circuit of the diesel engine 8.

A control panel comprises:

-   -   a start/stop button,    -   an operating mode selector (hot, cold ventilation),    -   a fault lamp,    -   an On lamp,    -   a display indicating the information useful to startup.

As soon as the ducts are connected between the unit 1 and the aircraft,the operator can start the diesel engine 8, or put the unit 1 otherwiseunder electrical tension, by action on the main start/stop button.

The diesel engine 8 is equipped with an automatic starting system.

As soon as an electrical voltage is present, the operator chooses theoperating mode of the unit 1 (hot/cold/ventilation). From this moment,the unit 1 can function in a completely automatic fashion.

After the starting of the main fan 10, its speed increases until thepressure sensor SP1 at the air outlet 6 detects that the overpressure ofthe air propelled is substantially equal to Pmax. As from this moment,the pressure sensor SP1 acts on the variator of the main fan 10 in orderto maintain the overpressure within limit operating conditions andmaintain the maximum possible air flow at the main fan 10.

In cold mode, the pressure reducing valves 20 adapt the flow rate ofrefrigerant to the flow rate of air thus controlled so as to maintainthe temperature of the air sensed by the temperature sensor ST3downstream of the exchanger/evaporator 12 within a set range and thesurface temperature of the exchanger/evaporator 12 sensed by thetemperature sensor ST4 at a value at a value such as for example justabove 0 degrees Celsius, preventing frosting of the exchanger/evaporator12. Frosting of the exchanger/evaporator 12 may cause a loss of air flowand lead to the stoppage of the unit 1.

The modification to the flow rate of refrigerant influences, inoperation, the aspiration pressure (low pressure of the refrigerationcircuit). According to the reading of this pressure by the pressuresensor SP3, the capacity of the compressor 17 can be adapted to maintainthis pressure within a set range ensuring optimum functioning of theunit 1.

The variation in the flow rate of refrigerant also influences the heatload to be given off at the condenser 18. To ensure stable functioning,the central automatic controller, by means of the frequency variator ofthe cooling fan 21 of the condenser 18, modifies the speed of the fan 21according to the reading of the high-pressure sensor SP4.

Likewise, for the exchanger/evaporator 12, the pressure reducing valve27 of the economiser circuit 24 adapts the diverted quantity of the mainflow of refrigerant in order to ensure correct supercooling of theliquid refrigerant before it returns to the pressure reducing valves 20of the exchanger/evaporator 12.

In heating mode, when it is present, the central automatic controlleruses firstly the complementary exchanger 14 for reasons of energysaving. By controlling the three-way valve of the cooling circuit of thediesel engine 8, the central automatic controller sends part of thecooling water flow from the diesel engine 8 to the complementaryexchanger 14 in order to heat the air propelled towards the appliance.

If the air temperature at discharge is too cold, the central automaticcontroller powers up the elements of the electric heating means byregulating the various stages up to the set value. This set value is amaximum value predefined according to the physical characteristics ofthe materials making up the unit 1, the connection ducts and the maximumpressures withstood by the aircraft onto which the unit 1 is connected.

When functioning in hot mode or cold mode, the central automaticcontroller manages the unit so that the latter functions continuouslywithout the unit 1 stopping in safe mode. Apart from the hardware safetydevices stopping the elements, the quantities that can stop the unitscompletely are:

-   -   general excess current,    -   excess current on the main fan 10,    -   high condensation pressure (in cold mode),    -   high pressure of the propelled air.

In the case of risk of general excess current, a pre-alarm thresholdbefore complete stoppage is detected. The central automatic controllerthen reduces the electrical consumption of the compressor 17 (in coldmode) or of the electric heating means 13 (in hot mode) by reducing thecapacity of these by action on their regulation.

In the case of risk of excess current at the main fan 10, the electricalload of the motor of the main fan 10 is reduced by reducing its rotationspeed.

In the case of risk of high condensation pressure, the refrigerationload of the compressor 17 is reduced by the central automatic controllerby action on its regulation.

In the case of risk of overpressure of the propelled air, the rotationspeed of the main fan 10 is reduced by means of the frequency variator.

These different situations lead to a reduction in the capacity of theunit 1 in order to maintain it in operation when the operatingconditions are more severe than expected without exceeding the safetylimits immediately stopping the unit 1.

Although the present invention has been described with reference tospecific example embodiments, it is obvious that various modificationsand changes can be made to these examples without departing from thegeneral scope of the invention as defined by the claims. Consequentlythe description and drawings must be considered in an illustrativerather than restrictive sense.

REFERENCES OF THE FIGURES

-   1 Air conditioning unit-   2 Chassis-   3 Wheels-   4 Body-   5 Air inlet-   5′ Filters-   6 Air outlet-   7 Discharge orifices-   7 a Valve-   7 b Valve-   7 c Valve-   8 Diesel engine-   9 Electrical generator-   10 Main fan-   11 Refrigerant circuit-   12 Exchanger/evaporator-   13 Electric heating means-   14 Complementary exchanger-   15 Treatment chamber-   16 Collection tank-   17 Compressor-   18 Condenser-   19 Parallel circuits-   20 Pressure reducing valves-   21 Cooling fan-   22 Horizontal refrigerant reservoir-   23 Dehumidifying filter-   24 Economiser circuit-   25 Main branch-   26 Secondary branch-   27 Pressure reducing valve-   28 Heat exchanger-   29 Radiator-   30 Engine compartment-   31 Fan-   A1 Intersection of flow rate/pressure curves V2 and E1-   A2 Intersection of flow rate/pressure curves V2 and E2-   A3 Intersection of flow rate/pressure curves V2 and E3-   B1 Intersection of flow rate/pressure curve E1 and Pmax-   B2 Intersection of flow rate/pressure curve E2 and Pmax-   B3 Intersection of flow rate/pressure curve E3 and Pmax-   D Air output-   DA1 Air output at point A1-   DA2 Air output at point A2-   DA3 Air output at point A3-   DB1 Air output at point B1-   DB2 Air output at point B2-   DB3 Air output at point B3-   E1 Flow rate/pressure curve of a first enclosure-   E2 Flow rate/pressure curve of a second enclosure-   E3 Flow rate/pressure curve of a third enclosure-   P Overpressure of conditioned air at air outlet-   Pmax Maximum overpressure-   SP1 Pressure sensor-   SP2 Pressure sensor-   SP3 Pressure sensor-   SP4 Pressure sensor-   ST1 Temperature sensor-   ST2 Temperature sensor-   ST3 Temperature sensor-   ST4 Temperature sensor-   V1 Flow rate/pressure curve of a first fan speed-   V2 Flow rate/pressure curve of a second fan speed-   V3 Flow rate/pressure curve of a third fan speed-   V4 Flow rate/pressure curve of a fourth fan speed

The invention claimed is:
 1. An air conditioning system, comprising: Anaircraft; and A mobile air conditioning unit disposed on the groundoutside the aircraft, the mobile air conditioning unit being forsupplying air conditioning to an interior of the aircraft while theaircraft is parked on the ground, the mobile air conditioning unitcomprising: an air circuit having an air inlet, a main fan, and an airoutlet, said air outlet being configured to be connected to the interiorof the aircraft parked on the ground through one or more flexible ducts;a refrigerant circuit located in the air circuit and comprising anindividual heat exchanger, a compressor and a condenser, said individualheat exchanger cooling the air by evaporating the refrigerant, saidcompressor compressing the refrigerant and said condenser condensing therefrigerant before it is returned to the individual heat exchanger, saidindividual heat exchanger comprising at least two parallel circuits,each of the at least two parallel circuits being individually controlledand being provided with an individual pressure reducing valve, said aircircuit further comprising: a temperature sensor downstream of the heatexchanger and connected to means for controlling said individualpressure reducing valves in order to regulate the flow rate ofrefrigerant to maintain the air temperature downstream of the heatexchanger within a set range; and a pressure sensor at the air outletconnected to means for regulating at least one of the speed and power ofthe main fan in order not to exceed a maximum overpressure at the airoutlet, and said refrigerant circuit further comprising a compressorsuction pressure sensor connected to means for regulating the mass flowrate of refrigerant aspirated by the compressor in order to maintain theaspiration pressure of the compressor within a set range.
 2. The airconditioning system according to claim 1, wherein said air circuitfurther comprises a temperature sensor connected to means for regulatingat least one of the speed and power of the main fan in order to controlits output so as to keep the temperature at the air outlet in a setrange.
 3. The air conditioning system according to claim 1, wherein saidcondenser comprises a cooling fan and a pressure sensor for therefrigerant connected to means for regulating at least one of the speedand power of the cooling fan in order to control the cooling of thecondenser so as to keep the pressure of the refrigerant in the condenserin a set range.
 4. The air conditioning system according to claim 1,wherein said individual heat exchanger further comprises a surfacetemperature sensor for the individual heat exchanger connected to saidmeans for controlling said pressure reducing valves in order to regulatethe flow rate of refrigerant so as to prevent the formation of frost onthe surface of the individual heat exchanger.
 5. The air conditioningsystem according to claim 1, wherein the refrigerant circuit furthercomprises an economiser circuit downstream of the condenser, saideconomiser circuit comprising a main branch, a secondary branch with apressure reducing valve, and a heat exchanger between said main branchand said secondary branch downstream of its pressure reducing valve, sothat, in operation, a secondary flow of refrigerant is diverted to saidsecondary branch, so as to be expanded in the pressure reducing valveand cool, through the heat exchanger, a primary flow of refrigerantpassing through the main branch so as to prevent the appearance of gasbubbles in the refrigerant upstream of the individual heat exchanger. 6.The air conditioning system according to claim 1, further comprising athermal engine, coupled to a generator for the electrical supply to theunit.
 7. The air conditioning system according to claim 6, wherein saidthermal engine comprises a cooling circuit with: a radiator, for coolinga cooling fluid for the thermal engine, a complementary heat exchangerplaced in said air circuit, downstream of said individual heatexchanger, for heating the air while cooling the cooling fluid of thethermal engine, and a three-way valve for directing the cooling fluidfrom the thermal engine alternately to the radiator or to thecomplementary heat exchanger.
 8. The air conditioning system accordingto claim 1, wherein said air circuit also comprises electric heatingmeans downstream of said individual heat exchanger, said electricheating means comprising a set of electric elements wired in severalstages, and a temperature sensor downstream of the electric heatingmeans connected to means for controlling said set of electric elementsin order to regulate the heating power so as to maintain the airtemperature downstream of the heating means in a set range.
 9. The airconditioning system according to claim 1, wherein said refrigerant is ahydrofluorocarbon.
 10. The air conditioning system according to claim 1,wherein said main fan is a centrifugal fan.
 11. The air conditioningsystem according to claim 1, wherein said compressor is a screwcompressor.
 12. The air conditioning system according to claim 1,wherein each of said at least two parallel circuits are supplied withthe air by the same main fan and wherein the individual heat exchangeris a single integrated unit comprising said at least two parallelcircuits.
 13. The air conditioning system according to claim 1, whereinthe individual heat exchanger further comprises a temperature sensor forsensing the temperature of the refrigerant at the outlet of each of saidparallel circuits, said temperature sensor being connected to said meansfor controlling said pressure reducing valves in order to regulate theflow rate of refrigerant so as to ensure the evaporation ofsubstantially all the flow of refrigerant in said individual heatexchanger.
 14. The air conditioning system according to claim 13,wherein said individual heat exchanger further comprises a pressuresensor for sensing the pressure of the refrigerant at the outlet of saidat least two parallel circuits, and wherein said pressure sensor isconnected to said means for controlling said pressure reducing valves inorder to correct the temperature sensed by the temperature sensor forthe refrigerant discharged from the same parallel circuit.
 15. An airconditioning method for air conditioning an interior of an aircraftparked on the ground, the method comprising: a) moving a mobile airconditioning unit on the ground to a position outside the aircraftparked on the ground; b) connecting the air conditioning unit to theinterior of the aircraft parked on the ground through one or moreflexible ducts; c) propelling ambient air into an air circuit, locatedin said air conditioning unit by a main fan; d) passing said ambient airthrough an individual heat exchanger located in the air circuit in orderto cool the ambient air by evaporation of a flow of refrigerantcirculating in the individual heat exchanger, said individual heatexchanger comprising at least two parallel circuits, each of the atleast two parallel circuits being individually controllable and beingprovided with at least one pressure reducing valve; e) propelling saidcooled air through an air outlet to the interior of the aircraft parkedon the ground through said one or more flexible ducts; f) compressingsaid refrigerant in a compressor downstream of the individual heatexchanger; g) condensing said refrigerant in a condenser downstream ofthe compressor; h) regulating the flow rate of refrigerant through eachof said pressure reducing valves so as to maintain the air temperaturedownstream of the individual heat exchanger in a set range by using atemperature sensor located downstream of the individual heat exchangerand which is connected to means for controlling each said at least onepressure reducing valve of the at least two parallel circuits; i)regulating the mass flow rate of refrigerant aspirated by the compressorso as to maintain the aspiration pressure of the compressor in a setrange by using a compressor suction pressure sensor connected to meansfor regulating the mass flow rate of the refrigerant; and j) regulatingat least one of the speed and power of the main fan so as not to exceeda maximum overpressure at the air outlet by using a pressure sensorlocated at the air outlet and connected to means for regulating at leastone of the speed and power of the main fan.
 16. The air conditioningmethod according to claim 15, wherein step j) comprises regulating atleast one of the speed and power of the main fan in order to maintainthe temperature at the air outlet in a set range.
 17. The airconditioning method according to claim 15, wherein step g) comprisescooling said condenser by air with a cooling fan at least one of thespeed and power of which is regulated so as to maintain the pressure ofthe refrigerant in the condenser in a set range.
 18. The airconditioning method according to claim 15, wherein step h) comprisesregulating the flow rate of refrigerant through each of said pressurereducing valves of the individual heat exchanger to ensure theevaporation of the flow of refrigerant in said individual heatexchanger.
 19. The air conditioning method according to claim 15,wherein step h) comprises regulating the flow rate of refrigerantthrough each of said pressure reducing valves of the individual heatexchanger to keep a surface temperature of the individual heat exchangerhigher than a minimum threshold in order to prevent the formation offrost on the surface of the individual heat exchanger.
 20. The airconditioning method according to claim 15, further comprising thefollowing steps: k) dividing the refrigerant flow downstream of thecondenser into at least a primary flow and a secondary flow, thesecondary flow being expanded in a pressure reducing valve of aneconomiser circuit, l) crossing the primary and secondary flows in aheat exchanger of said economiser circuit so that the secondary flowcools the primary flow, m) returning the secondary flow to the condenserand the cooled primary flow to the individual heat exchanger, and n)regulating the secondary flow so as to regulate the temperature of thecooled primary flow in order to prevent the formation of gas bubbles inthe refrigerant upstream of the individual heat exchanger.